A sensor unit such as a contact image sensor (hereinafter, referred to as CIS) used in an image forming apparatus includes a sensor array in which a plurality of sensors are arranged in parallel in a reading scanning direction (main scanning direction) and is mounted on a transport unit so as to detect transporting a sheet.
In the related art, the plurality of sensors in the sensor array output a reading signal (signal of a density value) of the sheet in the corresponding sensor in synchronization with each reference clock sequentially input to each sensor. Further, the sensors detect a front end and a rear end of the sheet based on the reading signal.
The reading signal output from each sensor, which is an analog signal, is subjected to A/D conversion so as to be converted into a digital signal and is transmitted to an upper level controller or an image processing unit in synchronization with the reference clock. The sensor unit according to the related art detects the front end and the rear end of the sheet by receiving the digital signal subjected to the A/D conversion. However, when a delay time occurs due to the A/D conversion, the sensor unit may not accurately detect the front end and the rear end of the sheet.